JPS5844237B2 - Nuclear reactor core fuel loading and operation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the design and arrangement of fuel within a nuclear reactor and a method of operating a nuclear reactor of such arrangement.

Nuclear fuel contains uranium and/or plutonium in suitable form.

For example, the fuel commonly used in water-cooled and moderated power reactors consists of uranium dioxide (UO2), of which about 0.
7-4.0% fissile U-235 and fuel parent U-238
It is a mixture of

During the operation of the reactor, the fuel parent U-238 is converted into fissile Pu-2
39 and Pu-241, the latter serving to maintain reactor power.

U-238 is also fissile, but only to high-energy neutrons in nuclear reactors.

In well-known commercial boiling water power reactors, such as those used at the Dresden Nuclear Power Plant near Chicago, Illinois, the nuclear fuel is typically
No. 365,371, sintered pellets are filled into elongated cladding tubes of a suitable metal such as a zirconium alloy to form fuel elements or rods.

The cladding sealed with end plugs serves to separate the nuclear fuel from the moderator/coolant and to prevent the release of fission products.

Such fuel elements are described, for example, in U.S. Pat.
No. 358, the fuel is arranged in groups and supported between upper and lower tie plates as separately replaceable fuel assemblies or bundles.

A sufficient number of fuel assemblies are arranged in a prismatic matrix to form a nuclear reactor core that undergoes a self-sustaining nuclear fission reaction.

The core is submerged in a fluid, such as light water, that acts as a working fluid and a neutron moderator.

A typical nuclear reactor is periodically reloaded with fuel of more than sufficient reactivity to maintain operation for an operating cycle that is often on the order of a year long. (referred to as the "fuel cycle interval").

At this point, the reactor is shut down and typically about H fuel assemblies are replaced.

Since the reactivity is excessive at the beginning of the operating cycle, a sufficiently strong control system is required to maintain the effective multiplication factor at unity during operation.

The control system conventionally includes a neutron absorbing material that acts to control the number of neutrons by non-fission-inducing capture or absorption of neutrons.

A typical control system has a mechanical control in the form of a plurality of control rods containing neutron absorbing material.

Control rods can be selectively inserted into spaces or gaps between fuel assemblies to control the reactivity of the reactor core, and thus the operating power level.

In known arrangements, such as those disclosed in U.S. Pat. No. 3,020,888, the control rod blades are cruciform in cross-section, thus dividing the blade "wings" of each control rod into four adjacent can be inserted into the space between the individual fuel assemblies.

A cluster of four fuel assemblies surrounding one control rod is called a core "cell."

(Suitable neutron absorbers and control blade drive mechanisms are described in the aforementioned US Pat. No. 3,020,888.

) The control system also includes a combustible neutron absorber, such as gadolinium, blended with a portion of the fuel.

The naturally occurring Gd-155 and Gd-157 isotopes are strong neutron absorbers, and absorption of neutrons converts them to isotopes of lower control value (neutron absorption capacity).

Using combustible absorbers in this manner reduces the need for mechanical controls and allows for improved power distribution through proper placement of the combustible absorbers.

In most cases, the combustible absorbent material is introduced into the fuel element as a mixture with a selected portion of the nuclear fuel.

The arrangement of combustible absorbent materials is described, for example, in U.S. Pat.
No. 839.

Furthermore, explanations about nuclear reactors can be found in, for example, ``Nuclear Power Engineer.''
g) JN, M, E 1-Waki l, McGr.
aw-Hill BookCom pany, I n
Seen in C., published in 1962.

Tubular fuel element cladding is typically 0.8M (0,03
(2 inches) thick and are exposed to relatively harsh operating conditions, including high pressures, high temperatures, nuclear radiation, and chemical fission product attack within the environment of a nuclear reactor core.

When the inserted control rod is withdrawn, the local power of the adjacent fuel element increases suddenly.

These sudden and large changes in the local linear power level (KW/ft) of the fuel result in large local stresses and strains due to the interaction of the fuel pellets with the cladding.

When the expanding separation edges of adjacent pellets (or adjacent sides of pellet cracks) bite into the cladding, the resulting local strain exceeds the ultimate strain in the cladding, causing it to crack.

Cracks in the cladding allow coolant to enter the fuel element and allow fission products to escape from the fuel element into the surrounding coolant.

This undesirable phenomenon is explained by the “pellet-cladding interaction J (Pellet-cla-adding 1
interaction = PCI).

Regardless of the magnitude of the increase in line power, there is a known fuel combustion dependent threshold below which cruding will not fail.

Among the various methods proposed to solve the pellet-crusting interaction (PCI) problem, a method has been used to condition the fuel so that it can withstand subsequent rapid power changes.

This method is described in US Pat. No. 4,057,466.

Briefly, according to this method, the rate of increase in output is set to, for example, 0. I K W, /f t-/hr or less is systematically controlled to alleviate the localized PCI generated pellet force.

Control the rate of power increase below a critical rate that results in cladding failure at an increase in local line power between the low value of PCIL and the desired maximum local line power level.

After such conditioning, it has been determined that relatively rapid output changes below the maximum adjustment level can be made without cladding failure.

A first disadvantage of this method is that such adjustment or readjustment requires a relatively long time, thereby reducing the time available for operation at the rated power level.

Also, in many practical operating situations, it is not possible to perfectly regulate the fuel near the inserted control rods.

Another phenomenon that accompanies nuclear reactor operation is the so-called “control rod history”.
(control rod history).

The effect of the presence of the control rod blades is to significantly reduce the rate of fissile fuel combustion or burn-up in the adjacent fuel, while limiting the conversion of the fuel parent U-238 to fissile Pu-239 to significantly lower levels. .

Therefore, when a control rod is pulled out, the
The power of exposed J (uncovered) fuel (i.e., fuel in the corners of the fuel assembly and control rod adjacent elements) increases more greatly when it is away from the control rod than fuel away from the control rod.

This control rod history effect is greatest in the fuel in the corner fuel elements of the fuel assembly adjacent to the control rod, and becomes more pronounced the longer the control rod remains next to the fuel.

Control rod history effects are greatest for control rod driven rod or followerless designs.

Another phenomenon that often occurs in boiling water reactors is axial steam void suppression.
upper-ession).

Boiling of the coolant in the individual channels in this type of reactor causes negative power feedback.

The reason is that the local reactivity of the fuel decreases with increasing steam voids.

By partially inserting the control rod into the bottom of the channel, the control rod suppresses boiling near the control blade and correspondingly reduces steam voids in areas of higher reactivity above the control blade. .

The reduction in boiling above a partially inserted control rod can result in severe power peaking, with the power peak exceeding the magnitude of the power in the channel when the control rod is fully withdrawn.

Early in the design and operation of power reactors of the type described above, control rod insertion and withdrawal processes and patterns were developed.

The basic solution is to periodically change and swap control rod patterns to try to distribute fuel burn, plutonium worming, and control rod history effects as evenly as possible among the fuel assemblies in the core. ing.

According to known control rod operating procedures for reactors of the type described above, the control rods are arranged in several alternating patterns, whereby one group of control rods is exchanged for another group of control rods during operation. I can do it.

There are usually two, three or four control rod patterns.
Control rods with a pattern of S or S are selectively inserted into the reactor core depending on the output shape and combustion reactivity control.

According to known control rod operating procedures, the core is operated in a predetermined control rod pattern for one energy production period.

The power is then reduced and the predetermined control rod pattern is replaced with another pattern.

Therefore, the control rod pattern will be changed 5 to 8 times during a one-year reactor operating cycle.

Such control rod patterns and pattern exchanges are detailed in US Pat. No. 3,385,758.

With known control rod operating procedures, most fuels experience adjacent control rod movement at power during their approximately four year stay in the core.

Such control rod motion is the result of combustion control, control rod pattern exchange, load following, xenon transient control, fuel conditioning, etc.

These operating variables undesirably increase the total number of control rod movements that the fuel experiences.

In addition, control rod pattern exchanges can excite spatial power distribution xenon transients, and control rod movement constraints based on thermal, hydraulic, safety, and fuel conditioning limits can undesirably complicate reactor operation and cause operator error. The possibility of

Known operating procedures therefore tend to reduce thermal and safety margins, increase manufacturing complexity, reduce availability, and increase the risk of fuel damage.

The problems encountered with known BWR operating methods of interchanging or swapping control rod patterns can be summarized as follows.

(1) In order to perform pattern exchange, the reactor power must be reduced while complying with PCI constraints.

In most cases, it takes as many as five days to bring the reactor back to full power after replacement, thus reducing reactor availability.

(2) Pattern exchange complicates reactor design and operation.

Since it is desirable to replace the control rods when the output drops for some other reason, it is difficult to establish a reactor operation plan, and even the same reactor operates at different fuel cycle intervals.

(3) Control rod replacement and associated power reductions result in spatial and non-spatial xenon transients that complicate reactor operation and make it difficult to meet PCI limits.

(4) Reactor operator learning curves are lengthened due to complex and interacting three-dimensional variables and constraints.

This increases the possibility that an operator will make a mistake that violates PCI or other constraints.

(5) All fuels except those located at the outer periphery of the core experience large linear power increases due to adjacent control rod movement during the fuel cycle interval between reloads.

The core periphery is usually the only area where fuel can be placed so that it does not experience adjacent control rod movement.

(6) All control rods other than the control rods located on the outer periphery of the reactor core must perform the dual functions of power shaping/combustion reactivity control and reactor shutdown.

Therefore, it is not easy to incorporate the special design characteristics required for these different functions into the design of control rods and control rod drive mechanisms.

(7) Applying automatic power distribution spatial shape control for load following or other purposes is very complex due to the large number of variables and their complex interactions.

Additionally, the following disadvantages have been observed from placing control rods next to high power or high reactivity fuels or next to undepleted combustible absorbent-containing strands.

Low steam void water above partially inserted control rods often causes peak local reactor power to reach these regions.

Thermal limits, such as transitions or departures from the nucleate boiling limit, are also adversely affected by control rods partially inserted next to highly reactive fuel.

Control rods inserted adjacent to undepleted combustible absorber-containing strands tend to distort absorber combustion, causing undesirable combustible absorber space transients that reduce the peak local power of the reactor. or require the fuel to be designed and manufactured into complex combustible absorbent shapes.

In achieving the present invention, the functions of the control rod are divided into a power shaping-reactivity control function and a shutdown function, and a specially designed fuel with low reactivity is maintained in the control cell to control the power shaping-reactivity control. Insert the control rod into this control cell during output.

In the initial core, the control cells are loaded with specially designed fuel assemblies with relatively low enrichment.

The uncontrolled cells are loaded with specially designed fuel assemblies that have relatively high initial enrichment and can be placed in the controlled cells during subsequent fuel cycle intervals.

As the reactor is put into operation and brought up to its rated output,
The control rods of the non-controlled cells are completely withdrawn, and the reactor is then controlled only by the control rods of the control cells during each operating cycle.

Therefore, only the specially designed low enrichment fuel in the control cell experiences adjacent control rod movement during the operating cycle.

This solution significantly simplifies core operation and avoids many problems associated with control rod replacement.

Shut down the reactor for reloading at the end of the operating cycle,
Typically, the control cell fuel assembly is evacuated (ie, removed from the reactor) or moved to a peripheral fuel assembly location in the core.

The lowest reactivity irradiated fuel assembly from outside the control cell is inserted into the control cell.

A new (i.e., unirradiated) fuel assembly is inserted outside the control cell (i.e., into a non-control cell), away from the control rods that should be inserted at rated power.

The fuel remains in the core for n core operating cycles (typically n is 4).

A fuel assembly typically spends (n
-1) remain in the open control cell for a cycle (but can be moved from one position to another in a non-controlled cell), are moved to the control cell in the final cycle of the stay in the core, and then in the core or removed from the reactor core or moved to the core periphery.

In some cases, unirradiated, specially designed high-reliability fuel can be inserted directly into the control cell at times other than the first cycle, in which case the fuel remains in the core for only m cycles.

However, m is the same as or different from the number n of residence cycles of uncontrolled cell fuel.

A core design that achieves separation of control rod functions and placement of only low reactivity fuel next to control rods for power shaping and reactivity control in accordance with the present invention is referred to as a "control cell core."

The life of the control blades is long because the stop control blades of the non-controlled cells are withdrawn at power-on and are therefore subject to less combustion, radiation damage and stress.

Therefore, it is easy to use expensive control materials in these control blades, especially in their upper portions, to maximize cold shutdown value and improve the reactor's fuel cycle combustion performance.

Furthermore, the drive mechanism for the control rods of the non-controlled cells can also be made relatively simple and inexpensive.

On the other hand, since the control rods placed in the control cell account for only about K of the total control rods in the reactor core, the control rods and drive mechanism of the control cell are optimal from the viewpoint of their primary use of power shaping and combustion reactivity control. It is easy to do.

As an optimization, a fine movement drive mechanism and a dray tip (tapered control value (iL) control blade) are provided.

These two features advantageously allow adjacent fuel to change power more slowly as the control rod moves.

Although microtransfer drive mechanisms are expensive and complex, they can be advantageously limited to use only in control cell locations.

It also becomes possible to use other control blade materials that ensure a longer lifetime and a more favorable neutron absorption spectrum.

For example, a long-life hafnium control material can reduce the output disturbance of the output shaping blade and extend the blade life.

By properly designing the fuel assemblies to be inserted into the control cell in the last or one last fuel cycle before removal from the core, the maximum operating capacity of the control cell core can be improved.

It is desirable to optimally design fuel assemblies for control rod history effects and the associated fuel burn periods during which these control rod history effects occur.

For the initial core, the fuel assemblies that are to reside in the control cells are specially designed for this purpose.

Also, the design of the fuel assemblies that are scheduled to be moved to the control cell during the first reload is different from the design of the fuel assemblies that are scheduled to be moved to the control cell during subsequent reloads.

In other words, the fuel assembly is custom designed according to its location in the core and its expected residence time in the core.

Reload fuel assemblies typically do not move to the control cell until the third or fourth cycle of their stay in the core, and the combustible absorber design for such reload fuel is different than for the initial core fuel.

Reload fuel assemblies can therefore also be optimally designed to derive maximum benefit from the separation of control functions.

In many applications, specially designed fuel assemblies can be used in the control cell because only about half of the fuel remains in the control cell during its in-core history.

A specially designed fuel assembly can be introduced into the control cell during the first, second, third, fourth or fifth cycle of its stay in the reactor, and in some cases may remain in the control cell for more than one cycle. You can keep it.

However, in a typical optimum, the fuel remains in the control cell only for the last cycle of its stay in the core, or one cycle before the last.

Manufacturing standards require that standard design fuel elements be used to form fuel assemblies and that the number of different standard fuel element types be minimized.

Therefore, fuel assemblies are designed to allow for the common use of as many standard fuel elements as possible while meeting design, performance and safety requirements.

Initial and combined reload fuel assembly designs for control cell cores can easily achieve these requirements, including reducing the number of standard fuel element types that need to be used to manufacture the required fuel assemblies.

The control cell core design of the present invention avoids placing undepleted combustible absorber-containing fuel assemblies next to the control rods inserted at power out; By reducing the number of different axial regions of combustible absorbent material within a strand-containing strand element, the design of the combustible absorbent-containing fuel element within a fuel assembly can be simplified.

Other control cell patterns may be used depending on specific core requirements and reactor design.

The main criteria are that the control cell pattern provides maximum core symmetry, and that there are enough control cells to ensure ease of reactor start-up, control of power profile and maximum excess reactivity, and load following of the grid used. It is to be.

The number and location of control cells may be changed at the beginning of a new fuel cycle interval if there are performance or other benefits to the change.

Separation of control functions and special core design make it possible to achieve the following basic criteria:

(1) Twisted material containing undepleted combustible absorbent material should not be placed next to control rods during significant output.

(2) Highly reactive fuel should not be placed next to control rods during steady state rated output.

(3) Minimize control rod pattern changes and control rod movements within one fuel cycle interval.

(4) Maintaining fuel that experiences large δ power changes due to adjacent control rod movement at a relatively low power during control rod withdrawal.

(5) Maximize the cold shutdown reactivity of the control rods in the local region of highest reactivity, and minimize the influence of control rod combustion on the cold shutdown control.

(6) Minimize local and global power disturbances due to control rods used for power shaping at rated power, and maximize control blade life for control blades that experience repeated significant neutron irradiation during power output. thing.

Applying these principles to control cell core design results in greater thermal margins, increased plant availability, reduced potential for operator error and improved safety, increased fuel reliability, improved fuel cycle economics, Benefits include simple fuel production and increased load following speed and range.

Furthermore, the reduction in the number of control rod pattern variables simplifies the design and increases the possibility of applying fully automatic direct output shaping control.

Next, the present invention will be explained in detail with reference to the drawings.

Although the present invention will be described as being applied to a boiling water type water-cooled moderation nuclear reactor, the present invention is not limited to this.

An example of a boiling water reactor is shown schematically in FIG.

The reactor comprises a pressure vessel 10 having a core 11 submerged in a coolant and moderator, such as light water.

The core 11 is surrounded by an annular shroud 12 and has a number of convertible fuel assemblies 1 spaced apart between an upper core grid 14 and a lower core plate 16.
Contains 3.

For the purpose of controlling the reactivity of the reactor core by the control rod drive mechanisms housed in the plurality of control rod drive mechanism housing tubes 17,
A plurality of control rods 18 can be selectively inserted between fuel assemblies 13.

A fuel assembly support member 19 is attached to each storage pipe 17,
Each support member 19 is formed with a socket for receiving the nose members 21 of four adjacent fuel assemblies.

A coolant passage or opening communicating with the coolant supply chamber 22 is formed in the nose member 21 and the support member 19 .

The coolant circulation pump 23 pressurizes the coolant in the supply chamber 22, thus transferring the coolant from the supply chamber 22 to the support member 19.
The fuel is forcibly fed upward into the fuel assembly 13 through the opening of the fuel assembly nose member 21.

As a result, a part of the coolant is converted into steam, and the steam passes through the steam separator-steam dryer 24 to the turbine 26.
reach steam-using equipment such as.

The condensed water formed in the condenser 27 is returned to the container 10 as water supply by the pump 28.

One control rod 18 and four fuel assemblies surrounding it occupy one fuel cell in the core.

A typical example of such a cell 29 is shown in the plan view of FIG.

Four fuel assemblies 13(1) to 13(
4) is laterally supported at its upper end by an upper core support grid formed by cross-connecting beams 30 and 32.

Each replaceable fuel assembly 13 is formed of a plurality of elongated fuel elements 34 supported spaced apart from each other between upper and lower tie plates (not shown) and surrounded by a tubular flow channel 36. be done.

Channels 36 serve to channel coolant upwardly between the fuel elements.

A more detailed illustration of such a fuel assembly can be found in the aforementioned US Pat. No. 3,689,358.

In nuclear reactors of the type described above, a batch of fuel that forms part of the reactor core is periodically reloaded.

Typically, a nuclear reactor is operated for 12 to 18 months between reloads, depending on fuel enrichment, reload batch size, and grid conditions used.

The operating interval between this reloading and the next reloading is referred to as a "refueling cycle J."

According to the invention, the fuel cells of the reactor core are arranged in a pattern of controlled cells and uncontrolled cells, and the controlled cells are arranged at least once each other.
separated by 1 uncontrolled cells.

Two examples that can be used as S patterns are illustrated by the plan views of one of the four symmetrical quadrants of the core in FIGS. 3A and 3B.

The fuel assembly 38 of the control cell is designated rCJ, and the fuel assembly 40 of the non-control cell is designated rNJ (C and N are C0n
trO1 and non-control).

Both of these patterns are % core symmetric with different patterns of control cells about the center of the core.

(While core symmetry is desirable, it is not a necessary condition for the control cell core arrangement described here.

) The number of control cells can be increased or decreased or the pattern can be changed to another between reload cycles.

For example, using the pattern of Figure 3A for one representative one-year reload cycle and using the relocated control cell in the position of Figure 3B during the next one-year reload cycle. I can do it.

The outer periphery of the core can be considered as a special region characterized by a high thermal neutron flux gradient, for example.

To facilitate the identification of fuel assemblies suitable for use in this region, the peripheral fuel assemblies are designated by "P" and the adjacent fuel assemblies are generally designated by rIJ.

For convenience of illustration, only one quadrant of the core is shown; however, if the core is symmetrical, one quadrant is sufficient to show the entire core arrangement.

Furthermore, according to the present invention, the control cell (hereinafter referred to as C cell) 38
is loaded with relatively low reactivity fuel, and the uncontrolled cell (
The fuel having a relatively high reactivity is loaded into the N cell 40.

For example, depending on the requirements of a particular case, the C cell may be approximately 0.711 to 1.2 W10 (
The N cell has a reactivity equivalent to the new fuel enrichment (typically 0.924 W10 for the initial core) (% by weight of fissile material);
It has a reactivity equivalent to an average fresh fuel enrichment of about 15W10.

The ratio of fissile material in the N cell to fissile material in the C cell is typically maximized according to design, operating, and fuel cycle conditions.

Furthermore, the outer peripheral fuel assembly P is loaded with fuel with a low reactivity equivalent to the fresh fuel enrichment of natural uranium (0,711W10), and the adjacent intermediate fuel assembly Load fuel with high reactivity equivalent to enrichment.

By using a combination of low-reactivity peripheral fuel and high-reactivity adjacent intermediate fuel in this way, neutron leakage from the core can be minimized, while the radial power distribution of the core can be flattened.
Improve margins on thermal performance limits.

According to the present invention, in order to bring the reactor from a stopped state to an operating state with the fuel arranged in this manner, the control rods of the N cells are sequentially pulled out from the reactor core starting from the outer peripheral part, and then the operation cycle is started. Control the power level and distribution using only the control rods of the C-cells and C-cells.

The control rod maneuver schedule of a C cell depends on, among other factors, the reactivity distribution of the fuel in the adjacent N cells.

(An example of a control rod operation plan for the C cell during the operation cycle will be shown later.)

) The general plan for reactor operation is
of IAEA Panel.

Vienna 1967 International
al Atomic Energy Agency (1
968) R.L.Crowther's paper in “Bu
rnup Analysis of Large Bo
i ling Water Reactors” (Combustion Analysis of Large Boiling Water Reactors).

The constant combustion power cycle initial to final states described in this paper are based on the “Haling distribution J”.
tribution).

When a nuclear reactor core has been operated for a sufficient number of uniform repetitive operating cycles, the core reaches "equilibrium" with respect to the number and enrichment of replacement fuel assemblies and their distribution within the core.

Such a core is therefore called an equilibrium core, and its operating cycle is called an equilibrium cycle.

Typically, an equilibrium core is reloaded every 71 years by replacing about a quarter of the total fuel assemblies.

Therefore, in an equilibrium core, the fuel assembly is 26,000 MWD
In the case of a typical irradiation output of about /ST (megawatt-day/standard ton), the irradiation stays in the core for about four years.

In an equilibrium core, in accordance with the present invention, the most burning or least reactive fuel assembly of the N cell is used in the C cell during the final cycle of that assembly's stay in the core.

In this example, the N-cell fuel is specifically designed to be placed in the C-cell during its final cycle and to exhibit maximum thermal margin and fuel reliability.

In other embodiments of the invention, special fuel assemblies are loaded into C cells as fresh fuel or after residence in some N cells for one to three reload cycles.

In practical operation, nuclear reactors rarely reach true "equilibrium."

Therefore, practical nuclear reactor core design requires the ability to adjust reactivity, reload batch size, control rod pattern, and reactor operating mode to accommodate actual variations in reactor operating conditions.

According to the present invention, to achieve this objective, repeating arrays of controlled and uncontrolled cells are maintained in the central portion of the core and low reactivity cells are maintained in the peripheral region to prevent neutron leakage from the core. The intermediate region between the central region and the peripheral region is minimized and allows the number and volume of fuel assemblies to vary.

Therefore, the number of C cells can change from one reload cycle to the next.

The ability to reload more than one fuel assembly provides further flexibility.

The fuel assemblies inserted during reloading are of type, average enrichment,
The combustible absorbent content and other design characteristics can be varied and can be constructed with fuel removed from previous reload cycles.

However, in a new reactor that has just entered operation, all of the fuel in the core is usually new.

The fuel assemblies used in such initial cores are designed and arranged to effectively mimic an equilibrium core, thereby facilitating movement from the initial core to an equilibrium core.

The application of the invention to initial and transition cycle operation is described below.

Figures 4A to 4G show examples of core arrangement used in the initial core and examples of fuel assemblies used therein.

The initial core arrangement shown in FIG. 4A includes different fuel assembly types Gi, Na, shown in FIGS. 4B-4F.

Using Nb, Ii and Pi.

In Figures 4B-4F, the fuel element position designated W is occupied by a non-fuel loading tube having lower and upper openings through which water moderator flows (e.g., U.S. Pat. No. 3.80
No. 2,995).

4th C. In Figures 4D and 4E, the fuel element designated BA has a portion of its fuel material mixed with a combustible absorbent material, such as gadolinium.

An example of the axial distribution of such a combustible absorbent material is shown in Fig. 4G.

Such a simple combustible absorber arrangement is suitable for the control cell core and its operation in accordance with the present invention.

This is because such an arrangement does not insert control rods near fuel assemblies containing significant unburned combustible absorbent material at power output.

Power distribution disturbances caused by the control rods and their interaction with combustible absorbent combustion are thus avoided.

Also, as shown in FIGS. 4C and 4D, particular fuel elements may or may not contain combustible absorbent material depending on the requirements of the particular core and the location of the fuel assembly within the core.

The fuel assemblies Ci, Na, Nb, Ii and Pi are the fourth
The arrangement in Figure A is arranged in a manner that maximizes the total power capability of the core throughout the initial operating cycle while meeting design safety constraints.

The outer peripheral region, which occupies about % of the core volume, is loaded with a low enrichment aggregate Pi (Fig. 4F) to minimize the leakage of neutrons from the core.

A highly enriched fuel assembly Ii (Figure 4E) is placed next to the outer assembly to flatten the core radial power distribution and improve thermal performance.

Fuel assembly Ci (Figure 4B) is designed for use in the C cell throughout the initial operating cycle.

In addition to the low average enrichment, the enrichment distribution of the fuel elements in this fuel assembly is designed to place the lowest enrichment fuel elements in the closest fuel element locations to adjacent control rods.

The fuel element distribution is optimized for long-term power operation with control rods inserted adjacent to the fuel.

This arrangement allows the fuel assembly to accommodate the required long periods of time from being adjacent to an inserted control rod blade to subsequent control rod withdrawal, and to provide associated local power peaking based on the history of previous control rod insertions. can be compensated.

(Furthermore, the fuel assembly Ci can also be used as a substitute for the fuel assembly Pi at the outer periphery of the core.

) The fuel assemblies Na, Nb and Ii are designed to initially stay in the N cell and, after sufficient irradiation, to be finally used in the C cell.

For example, fuel assembly Na becomes C cell in the second cycle, fuel assembly Nb becomes C cell in the third cycle, fuel assembly ■
1 can move to the C cell in the fourth cycle.

One of the design features of these fuel assemblies that enables such use is the unique fuel enrichment distribution that ensures final cycle operation when control rods are left inserted for extended periods of operation.

In another approach, a fresh (unirradiated) Ci-type fuel assembly can be loaded into the control cell for each reload cycle.

The fuel elements of these fuel assemblies can be designed with extra protection from large delta powers resulting from adjacent control blade movement, e.g., coated barriers as described in U.S. Pat. No. 3,925,151. I can do it.

As mentioned above, in the operation of the control cell core, all control rods except the control rods of the C cell are withdrawn as the reactor is brought to the power state.

Therefore, the operation at output is controlled by the control rod of the C cell.

The C-cell control rod maneuver schedule is extremely simple, especially compared to conventional control rod maneuver schedules.

The special pattern of fuel assemblies shown in FIG. 4A facilitates reactor start-up and increases operational flexibility.

The fuel assembly Na (FIG. 4C) with the second lowest reactivity is placed in another control cell location symmetrical to the core.

During start-up, the control rod in the Na position has a steam void coefficient,
It is used to partially control the Doppler coefficient and xenon reactivity and is the last group of control rods to be withdrawn before the control rods are used only in the Ci position to control core operation.

This minimizes localized power off the tips of the control blades that are moved at low reactor powers;
Nuclear reactor operation can be started more quickly and reliably.

During the next reload cycle, change the position of the control cell (Ci) to N
a position, the fuel assembly from the C1 position can be taken out or moved to the outer peripheral position Pi, the fuel assembly at the Nb position can be moved to the Ci position, and the new fuel assembly can be moved to the empty Nb position. Place it in

The next reload cycle returns the control cell to the Ci position shown in Figure 4A.

This combined reloading and operation plan reduces the number of fuel assemblies that must be moved during reloading.

An integrated balanced reload plan using a similar plan is described below.

As an example, the C cell control rod maneuvering plan will be described in general terms with reference to FIG.

In FIG. 5, the C cell control rod is shown as one component of a group of eight cells.

Using the group designation of FIG. 5, the selection of C cell control rod patterns throughout the cycle is typically as follows.

(1) Insert the control rods of groups 1, 2, and 8 into relatively deep positions.

These control rods are used to compensate for reactivity changes during the cycle until approximately % of their length is withdrawn from the core.

These control rods are then completely withdrawn from the core.

(2) Group 3 control rods in intermediate position and groups 1, 2 and 8
control rods to bring the core into a critical state, peak the entire radial core power distribution toward the center of the core, and change the magnitude of the central peak to at least the central peak of the Hering burnup distribution described above. Make equal.

(3) Insert the control rods of group 7 from a shallow position to a deep position.

These control rods serve a dual purpose. These control rods are used to shape the radial core power distribution during the sub-period of the cycle in which the control rods of groups 1 and 2 are deeply inserted.

Additionally, these control rods provide the necessary axial power shaping to maintain an acceptably defined power density in the high reactivity fuel assemblies adjacent to the circumferential assembly.

Additionally, during post-charging start-ups and other conditions where operational flexibility is required, deep insertion of group 7 control rods may cause fuel to dislodge the blade tips when the control rods are partially withdrawn. The local power density of the pellet-cladding interaction (PCI
) L threshold limited power so that these control rods can be moved from a deep position at rated power.

(4) Groups 4 and 5 control rods are used only for axial power shaping during the portion of the cycle in which Groups 1, 2, and 8 control rods are inserted.

During this period, control rods are inserted only as needed to maintain an acceptable power density margin for adjacent high reactivity fuel assemblies.

When the control rods of groups 1, 2 and 8 are completely withdrawn, group 4
and 5 control rods are inserted to take over the role of reactivity control for the remainder of the cycle.

This replacement of Groups 1, 2 and 8 control rods with Groups 4 and 5 control rods is sometimes done to minimize peak power density in the C cell fuel assembly.

Because the C cell fuel assemblies in cells in groups 4 and 5 have no control rod history above their center planes at the time of replacement, these local power beaks would be similar to those in groups 1, 5, and 5 if no replacement occurred.
This is lower than the peaking that a 2 and 8 control cell fuel assembly would exhibit.

(5) Finally, Group 6 control rods may be inserted only when necessary to maintain acceptable power densities in high reactivity fuel assemblies, or similar to the Group 7 control rod application described above. Achieve deep control and full power operation flexibility.

At the end of one operating cycle, the reactor is shut down for reloading.

Usually, the outer fuel assembly is removed from the core.

The C-cell assemblies may be removed from the C-cell and removed from the core, or optionally moved to a peripheral fuel assembly location for use in the core for one more cycle.

The C cell is reloaded with the fuel assembly having the highest combustion and lowest reactivity from the N cell.

(High combustion assemblies from N cells can also be moved to outer fuel assembly positions.

) Insert a new fuel assembly into the N cell (preferably in a scatter loading manner).

These new aggregates have the appropriate enrichment to provide the necessary reactivity throughout the next operating cycle.

An example arrangement of a core reloaded in accordance with the present invention is shown in FIG. 6A.

The subscripts O to 3 of the fuel assembly identification codes (N, C, P) indicate the number of irradiation cycles that the fuel has received up to the time of this reloading.

FIG. 6B shows another pattern exhibiting a reactivity distribution similar to the initial core design of FIG. 4A.

The fifth cycle fuel located at the outer periphery (P4 position) is typically the fourth cycle outer circumferential fuel from the previous cycle, but may also be fuel moved from the control cell position.

To repeat this pattern at equilibrium, P4. Remove the C3 and/or P33 fuel assembly, move the N2 and/or C3 fuel assembly to the P position, and move the N2 fuel assembly to the C
3 position, move the N□ fuel assembly to the previous N2 position, and charge new fuel into the empty N1 position.

Figure 6C shows another charging pattern similar to Figure 6B, but in this case the control cells are placed at a different location with respect to the center of the core to achieve a core arrangement similar to that of Figure 3B. .

It should be noted that the control cell in the configuration of FIG. 6C is in the same location as the N2 fuel cell in the configuration of FIG. 6B.

Since the N2 fuel assemblies are scheduled to be used in the quantity control cell for that last cycle, the number of fuel assemblies that must be moved during reloading is as follows: This can be minimized by sequentially changing from the control cell pattern of , to the pattern of FIG. 6C in the next reload cycle, and to the pattern of FIG. 6B in the next reload cycle.

This alternative spreads the use of control blades and combustion over a larger number of control rods, thus requiring approximately twice the number of specialized long-life control rods for power shaping and reactivity control within the control cell. .

FIG. 7 shows the new fuel N in the core configuration of FIGS. 5, 6A, and 6C.

An example of a reloading fuel assembly used as a fuel assembly is shown below.

A notable feature of this design is the placement of relatively low enrichment fuel elements at the control rod corners of the fuel assembly, which minimizes control rod history effects and moves the assembly into N cells. It can be used in the C cell position after irradiation.

Special fuel elements may be placed at fuel element locations that are susceptible to reloading or damage from power changes or pellet-clad interactions within the initial core fuel assembly, such as the reloaded fuel assembly location 3°4 in FIG. , 5 and 7.

Such fuel elements can be provided with a protective coating barrier, for example a layer of copper or pure zirconium that protects the coating from attack by fission products.

Such damage resistant fuel elements may include annular fuel pellets and/or fuel pellets containing added plasticizers.

It should be noted that fuel assemblies according to the present invention may contain greater or lesser amounts of combustible absorbent material depending on core requirements and the location of the particular assembly within the core.

FIG. 8A shows examples of C cell control rod patterns at various doses E throughout the operating cycle of the balanced core of FIG. 6A.

The control rods are operated in groups, and the control rod groups 1 to 8 are the same as shown in FIG.

The number within the square representing the C cell indicates the cell's percentage of the total depth insertion of the control rod.

Thus, 100 indicates full insertion and 0 indicates full withdrawal.

Control rods other than the control cell are not used for operation during significant power output, and movement of control rods in the control cell is minimized during reactor operation.

FIG. 8B shows the relative power adjacent to the control rod as a function of bottom entry control rod position for a core of conventional design and operation.

This figure shows the effect of reduced steam void injection at the top of the core, resulting in high peak power even if the control rods are inserted at the bottom of the core.

FIG. 8C shows a similar curve of relative power versus control rod position for a core designed and operated according to the control cell core concept of the present invention.

Notably, the peak power adjacent to the control blades is significantly reduced compared to the conventional core in Figure 8B, and power peaks are less likely to occur above or below the core, without increasing power peaking. The control blade can be inserted deeper into the core, and the fully withdrawn position of the control blade reduces power over about 80% of the top of the fuel.

FIG. 8D illustrates the effect of control rod movement within a control cell of the present invention on high reactivity fuel within a non-control cell.

Compared to the high reactivity fuel power distribution trend of the conventional design (Figure 8B), the power during control blade movement (KW,/ft,
), and with this fuel the axial power peak can be better controlled by partial insertion of the control cell blades,
The transition from one control rod state to another is smoother, the power increase at the top of the core is significantly smaller, and peak reactor local power is reduced.

Therefore, in the control cell operation mode according to the present invention,
Inserting the control blade approximately 17% into the bottom of the core does not result in a significant peak power increase in either the control cell fuel or the high reactivity fuel outside the control cell or non-control cells.

Additionally, by inserting the control blades of the control cells into the central core location, the peak fuel output of adjacent uncontrolled cells is significantly reduced.

By using these trends, a more appropriate output shape can be obtained with the control cell design of the present invention than is remotely related to conventional control methods that have been applied hitherto.

An important feature of the control cell core of the present invention is the functional separation of the C cell and N cell control rods.

C-cell control rods are used for core power level and shaping control, making up only about approximately of the total control rods.

The main function of the remaining control rods, ie, control rods other than the C cell, is to shut down the reactor.

By minimizing this functional separation and the number of control rods used for core power control, we modified the design of the control rods and their drive mechanisms so that all control rods and drive mechanisms have dual functions of power shaping and shutdown. The control rods and drive mechanisms can be configured to perform specific functions in a manner that would not be practical if the control rod and drive mechanism had to be performed.

For example, it is desirable for the control rods of a C cell to have a long life and for these control rods to change the local power of the adjacent fuel gradually, rather than abruptly, as they move.

An example of a control rod 181 and control rod drive mechanism 171 suitable for use in a C cell is shown in FIGS. 9A-9D.

The control rod 181 is formed by connecting an upper casting part 42 in which a handle 44 is formed and a lower casting part 46 with a center post 48 having a cross section.

A connecting socket 54 for attachment to a speed limiter 50, a guide roller 52, and a drive mechanism 171 is formed in the lower casting portion 46.

Guide rollers 55 are also attached to the upper casting 42 to support the control rods 181 laterally between the fuel assemblies.

A U-shaped sheath is attached to the center post 48 and the upper casting part 42.
and attached to the lower casting 46 to form the four blades 56(1) to 56(4) of the control rod.

A plurality of neutron absorption rods 5B are accommodated in each of the blades 56(1) to 56(4).

Typically, the absorber rod 58 is made of a suitable neutron absorbing material, such as natural boron carbide (B4C) powder 6, as shown in FIG. 9B.
It is formed from a sealed tube 60 containing 2.

The column of powder 62 is divided by a series of spaced balls 64 which are restrained from movement by circumferential crimps 66 in the wall of tube 60.

This preemption prevents voids from forming in the columnar body of powder 62, and also prevents all powder from being lost from the absorbent rod in the event that a portion of the absorbent rod is cracked.

It is desirable for a control rod located within a control cell to have minimal impact on adjacent fuel, thus minimizing local power changes when the control rod blade is withdrawn.

This means that the control rod blade has a "gray chip".
tip) This can be achieved to some extent by providing J.

The gray chips drain the water moderator and absorb fewer neutrons than the stronger control material elsewhere on the control blade.

It is also desirable for control blades located within the control cell to have a long life.

The tips and side edges of the control blade vanes absorb the greatest number of neutrons and these limit the control blade life.

The life of the control blade can be extended by selectively using long-life materials in the high capture regions of the control blade that do not produce gases or other damaging reaction products from neutron capture.

The features of adapting the control rod 181 for use in the C cell are as follows.

Control rods 181 can be formed with long life or "gray chips" in a variety of ways.

(Here "gray tip" means that the control strength increases gradually from the top or end of the handle.

) As shown in FIG. 9A, the handle 44 is wider and longer than normal and is made of, for example, stainless steel, which is a neutron absorbing material but has lower control strength than boron.

The enlarged handle 44 therefore drains water accordingly;
Transfers control strength from the tip of the control rod to the main body.

An alternative or additional method of forming a gray tip long life control rod 181 is shown in FIG. 9C.

In this example, the gray chip length is increased by filling the tubes 60 of the absorption rods 58 in the blades 56(1) to 56(4) with the absorbent material 62 so that its height gradually decreases toward the outer edge of the blade. Obtain a life control rod.

An arrangement that provides both a gray tip effect and long controlled blade life is shown in Figure 9D.

In this arrangement, instead of boron, a suitable neutron absorbing material, e.g.
to K and, in the case of one or more outer absorption rods, their entire length.

Because the control blade neutron flux gradient is steep, the control blade tip and outer absorption rod have the highest incident neutron flux, the highest neutron capture rate, and therefore the shortest lifetime.

Hafnium used in these locations provides much longer blade life than B4C.

The reason is that hafnium does not form damaging reaction products, including gases produced from B and C.

Although hafnium is less sensitive to thermal neutrons than B4C, it is a strong absorber of ebithermal neutrons.

Therefore, hafnium absorbs less thermal neutrons, but competes more with U-238 for evitothermal neutrons.

As a result, U-238 diversions are reduced and therefore control rod history effects are reduced.

For hafnium, the use of C-cell control rods, which increases costs, will be compensated.

The reason for this is that the C cell control rods are a minority of the control rods in the reactor core, and they have a very strict responsibility.

The control rod design of FIG. 9D minimizes cost by localizing expensive hafnium material only to areas of the control blade where this material is needed to extend control blade life.

Finally, in order to reduce the rate of change in local power in adjacent fuel when moving the control rod, the control rod drive mechanism
71 (FIG. 9A) is preferably of the micro-movement type.

A suitable fine movement lid scram 7 drive mechanism is disclosed in U.S. Pat.
No. 734,824.

The control rods outside the C cell, ie, the control rods of the N cell, are withdrawn when the reactor core is in power operation.

These control rods therefore have a long life and can be made of more absorbent, more expensive materials to maximize cold shutdown value.

An example of the control rod 182 and drive mechanism 172 used in the N cell is shown in FIG.

The general structure of the control rod 182 is similar to that of the control rod 181 shown in FIG. 9A.

Therefore, the general description will not be repeated. In cold conditions, for example in boiling water reactors, the fuel near the top of the core is highly reactive.

Therefore, in N-cell control rods, it is necessary to use more expensive and stronger neutron-absorbing materials only in the top K or % of the absorber rod.

Therefore, as shown in FIG.
4) contains boron carbide (B-10) in the upper % and ordinary B4C in the remaining lower % of its length.

Finally, control rod drive mechanism 172 can perform step and scram movements with relative ease.

A suitable example of such a drive mechanism is U.S. Pat.
, No. 887.

What has been described above is a design and arrangement of fuel and control rods for a nuclear reactor core, and a method of operation that greatly simplifies this core design and its operation and reduces the risk of fuel damage.

Automated control operations are made easier by significantly reducing the number of control rods that must be manipulated during output and by providing a more precise and simpler control rod pattern.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing a water-cooled and moderated reactor and its steam usage system, Figure 2 is a diagrammatic plan view of the fuel cells in the core, and Figures 3A and 3B are control cell arrangements, respectively. Diagrammatic plan view of the H section of the core showing different examples of
Figure A is a diagrammatic plan view showing the control cell core arrangement for the initial core; Figures 4B to 4F are arrangement diagrams of fuel assemblies Ci, Na, Nb, Ii, and Pi used in the arrangement of Figure 4A; Fig. 4G is a distribution diagram of combustible absorbent material in the fuel element, Fig. 5 is an explanatory diagram of group designations of control rods in the control cell, and Fig. 6A to 6C.
FIG. 7 is a plan view showing an example of a reloading balanced core design; FIG. 7 is a layout diagram showing an example of a reloading fuel assembly; FIG. 8A is a plan view showing an example of a reloading fuel assembly; FIG. An example of the pattern (see Figure 5 for group designation) is shown, and 8B
The figure is a curve diagram showing the local power variation of a high reactivity fuel assembly in a conventionally designed core for various positions of the control rods adjacent to the assembly (conventional Fig. 8C is a curve diagram showing the local power variation of a fuel assembly in a control cell of the present invention for various positions of the control rods within that control cell. Fig. 8D shows the local power change of the high reactivity fuel assembly in the non-controlled cell of the present invention compared to that in the adjacent control cell. Curve diagrams showing various positions of control rods (
Figures 9A-9D illustrate the control rods and drive mechanism used in the core control cell;
Figure A is a perspective view of the control rod and drive mechanism, Figure 9B is a sectional view of the absorption rod, Figure 9C is an example of the arrangement of the absorber in the control blade, and Figure 9D is another example of the arrangement of the absorber in the control blade. An example of the arrangement is shown in FIG. 10, which is a perspective view showing control rods and drive mechanisms used in non-controlled cells of the reactor core. 11... Core, 13... Fuel assembly,
18... Control rod, 29... Fuel cell,
34...Fuel element, 56...Blade, 58...Absorption rod, 171°172...
・C, N cell drive mechanism, 181, 182...C
, N cell control rod.

Claims (1)

[Scope of Claims] 1. A nuclear reactor core having a plurality of cells, each cell including a plurality of individually convertible fuel assemblies surrounding a selectively insertable control rod, and capable of continuous periodic operation. When loading and operating a nuclear reactor core in which the reactor is stopped every cycle and a part of the fuel assembly in the reactor core is replaced, (1) two types of cell patterns in the core;
(2) selecting a pattern consisting of a set of controlled cells and a set of non-controlled cells, where the controlled cells are separated from each other by at least one non-controlled cell; (2) a design specifically configured for use with said controlled cells; (3) placing a relatively low reactivity fuel assembly in a control cell in the control cell; (4) the control rods of the non-controlled cells are substantially withdrawn from the reactor and the high reactivity is sufficient to allow the fuel assembly to remain in the core for n operating cycle hours; (5) operating the reactor core for an entire operating cycle while controlling the power level of the reactor core by selectively inserting control rods of at least some of the cells into the reactor core; shutting down the reactor for loading; (6) removing all fuel assemblies from each of the control cells; (7) removing all fuel assemblies from each of the control cells;
(8) a number of irradiated fuel assemblies at least equal to the number of irradiated fuel assemblies that were moved to said control cell; , inserting unirradiated fuel assemblies with an enrichment level that exhibits sufficient reactivity for n operating cycles in the reactor core into the non-controlled cells; (9) following steps (4) to (8), continuously A nuclear reactor core fuel loading and operation method having the above-mentioned steps, in which the core is sequentially operated in an operation cycle in which 2. The method according to claim 1, wherein the core is operated in such a way that a blade of a partially inserted control rod is not inserted next to a high-reactivity heating material. 3. Some of the fuel assemblies contain combustible absorbent material, and the operation of the core is controlled by the use of partially inserted control rod blades adjacent to strands containing combustible absorbent material. Claim 1 or 2 made so as not to be inserted into
The method described in section. 4. said unirradiated fuel assembly inserted into said uncontrolled cell comprises a plurality of elongated, spaced apart or parallel fuel elements arranged in a four-sided array, with two adjacent sides of the assembly located within the reactor core; selected to be located adjacent to the blades of the control rods; a central group of fuel elements of the assembly containing relatively high enrichment fuel; and a middle group of fuel elements having an intermediate enrichment less than the high enrichment. the peripheral group fuel elements contain fuel with a low enrichment lower than the intermediate enrichment; the corner fuel elements contain fuel with an enrichment lower than the low enrichment; the predetermined adjacent side one corner fuel element located at a corner defined by contains a lower enrichment of fuel than every other fuel element of said assembly; some fuel elements of said assembly contain combustible absorbent material; 4. The method of claim 3 further comprising: at least one moderator flow rod extending between fuel elements of said center group. 5. Modify steps (7) and (8) to remove only some fuel assemblies from at least some control cells and to remove these fuel assemblies from uncontrolled cells that have been irradiated for (n-1) cycles. 5. A method as claimed in any one of claims 1 to 4, in which fuel assemblies from cells are replaced. 6 Modifying steps (7) and (8) to remove at least some fuel assemblies from at least some control cells and converting these fuel assemblies into relatively low reactivity unirradiated fuel assemblies. Any one of claims 1 to 4 exchanged with
The method described in section. 7. Modify steps (7) and (8) to remove only some fuel assemblies from at least some control cells and cycle these fuel assemblies (n-1) in said non-control cells. Claims 1-4 Replacement with a relatively low reactivity irradiated fuel assembly that has been irradiated for fewer cycles
The method described in any one of paragraphs. 8. Transferring only fuel assemblies from said non-controlled cells to said controlled cells that include means for increasing the resistance of fuel elements to pellet-cluttering interactions.
The method described in section. 9 further selecting a second control cell set including a fuel assembly of fuel whose reactivity is greater than the reactivity of the fuel in the control cells and smaller than the reactivity of the fuel in the non-control cells; Claims 1 to 8 include the step of using control rods at the start of reactor operation and while approaching full power operation, and maintaining the control rods of the second control cell in a withdrawn state during full power operation. The method described in any one of the above. 10. Claims 1 to 9 further comprising the step of arranging a control rod having a gray chip in the control cell, wherein the gray chip gradually reduces control strength along a portion of the upper end of the control rod. The method according to any one of the above. 11. Claims 1 to 10, characterized in that hafnium is disposed in the upper and side portions of the control rod of the control cell, and the remainder of the neutron absorbing material is within the control frame of the control cell.
The method described in any one of paragraphs. 12. The method according to any one of claims 1 to 11, comprising the step of arranging a control rod in the non-controlled cell, the control strength of which is stronger in the upper part than in the lower part. 13. Any one of claims 1 to 12, including the step of filling boron carbide (B-10) in the -H part of the control rod of the non-controlled cell and filling natural B4C in the lower part. The method described in Section 1. 14, the fuel assembly disposed in the control cell in step (2) includes low enrichment fuel in the fuel element of the assembly adjacent to the control rod channel of the control cell, but does not include combustible absorbent material;
14. A method as claimed in any one of claims 1 to 13, specifically designed to withstand movement of adjacent control rods. 15 A nuclear reactor core having a plurality of individually replaceable fuel assemblies surrounding selectively insertable control rods, the reactor core having a plurality of individually replaceable fuel assemblies, the reactor being shut down after each successive periodic operation cycle, and refueling the reactor core. When loading fuel and operating a nuclear reactor core in which a part of the assembly is replaced, (1) selecting a cell pattern including a first control cell set, a second control cell set, and a non-control cell set for the reactor core; The first set of control cells has a sufficient number to enable the control rods to control the reactor core at power output, and the cells of the first set are separated from each other by at least one cell of another cell set, and the second set of control cells have a sufficient number of control rods to control the core at power, and the cells of the second set are separated from each other by at least one cell of the other cell set;
(2) arranging the cells of the set at cell positions in the core different from the cells of the first control cell set, and arranging the uncontrolled cells of the uncontrolled cell set between the cells of the first and second controlled cell sets; arranging fuel assemblies of fuel with relatively low reactivity in the first set of control cells; (3) reducing the number of fuel assemblies of unirradiated fuel with sufficient reactivity for the stay time in the core for n operation cycles; (4) an average intermediate between the average cell reactivity of the cells of the first controlled cell set and that of the non-controlled cells; disposing a fuel assembly exhibiting cell reactivity in the second set of control cells;
(5) irradiating at least some of the fuel assemblies disposed in the second control cell set for (n-2) operating cycles; and (5) controlling the cells of the second control cell set and the control rods of the non-control cells. (6) operating the reactor core through one operating cycle while controlling the reactor core by substantially withdrawing it at power out and selectively inserting the control rods of at least some cells of said first control cell set; (7) removing at least some of the fuel assemblies of each cell of the first control cell set and placing them at a higher reactivity level; (8) removing at least some of the fuel assemblies of the non-controlled cells;
(9) replacing these with unirradiated fuel assemblies; 2 control cell sets, thereby minimizing movement of the fuel assembly during reloading,
Then (1) the cells of the first control cell set and the control rods of the non-control cells are substantially withdrawn at power out, and the control rods of at least some of the cells of the second control cell set are selectively inserted and the reactor is A method of fuel loading and operating a nuclear reactor core having the above steps, in which the reactor core is operated throughout the next operating cycle while the reactor core is controlled.In step 16 (7), replacing the cells of the first control cell set. the fuel assemblies include at least some fuel assemblies that have been irradiated for (n-2) operating cycles, thereby placing low reactivity fuel in the cells of the first control cell set during the next subsequent operating cycle; and controlling the reactor core by selectively inserting the control rods of the cells of the first control cell set while the control rods of the cells of the second control cell set and the control rods of the non-control cells are substantially pulled out from the core at the time of power output. 16. The method according to claim 15, wherein in step (8), the fuel assemblies placed in the non-controlled cells are irradiated and unirradiated for (n-3) cycles. A patent that further includes a step of arranging a fuel assembly of relatively low reactivity fuel at the outer peripheral fuel assembly position of the core. 18. The method according to any one of claims 15 to 17. 19. Further, a fuel assembly of a relatively highly reactive fuel is placed as an intermediate fuel in the core between the outer peripheral fuel assembly position and the cell set. 19. The method of claim 18, further comprising the step of placing at least some of the fuel assemblies at the intermediate fuel assembly location into a non-controlled cell location during reloading. 21. The method of claim 19, wherein at least some of the fuel assemblies located at the outer fuel assembly location are irradiated and remain at the outer location during a final operating cycle in the core. 21. A method according to any one of claims 18 to 20, further comprising the step of placing control rods having gray tips in cells of the first and second control cell sets.
Claims 15 to 21, wherein the gray chip gradually reduces the control strength along a part of the upper end of the control rod.
The method described in any one of paragraphs. Claim 15, wherein hafnium is disposed in the upper and side portions of the control rods of the cells of the first and second control cell sets, and the remainder of the neutron absorbing material in the control rods of the control cells. 23. The method according to any one of items 22 to 22. 24. The method according to any one of claims 15 to 23, comprising the step of arranging a control rod in the non-controlled cell, the upper control strength of which is stronger than the lower control strength. b. Any one of claims 15 to 24, including the step of filling boron carbide B-10 in the -H part of the control rod of the non-controlled cell and filling natural B4C in the lower part. The method described in.